Yanmar 3GM Compression Test

[Old Bumbulum]

I hope this is the right place to post this question.

I have just bought a boat with a 3GM engine and while the injectors are being cleaned I want to do a compression test. I borrowed a testing kit but none of the adaptors are anywhere close to fitting the engine.

Does anyone know if a special adaptor is available or how to do a comp test on a 3GM short of finding a machine shop to turn up a bespoke adaptor, a sixty quid job or so?

Thanks.

 

[RichardS]

I hope this is the right place to post this question.

I have just bought a boat with a 3GM engine and while the injectors are being cleaned I want to do a compression test. I borrowed a testing kit but none of the adaptors are anywhere close to fitting the engine.

Does anyone know if a special adaptor is available or how to do a comp test on a 3GM short of finding a machine shop to turn up a bespoke adaptor, a sixty quid job or so?

Thanks.

There must be adaptors for Yanmar engines but I'm not sure where to find them. You could start by calling the manufacturer of your tester and see what they suggest.

Richard

 

[Old Bumbulum]

I gather that compression tests, though much talked about are rarely if ever done in practice!

 

[robbieg]

Normally with these engines you need an old injector to cannibalise to connect your compression tester to. Its either that or the Yanmar adaptor at some ridiculous price (if you can even get one now). I have done it in the past so it can be done but most folk will isolate each cylinder with decompressors and see how easy it is to turn by hand using the flywheel pulley. This helps identify a weak cylinder. If they are all pretty similar in terms of effort to turn and it takes a bit of effort compression is probably reasonable. Leave alone would be my view unless you having difficulty starting or something that makes you think you have a problem with the rings/bore (e.g. excessive old consumption). Hope this helps.

 

[Tranona]

Another cause of low compression is a bent con rod. There are instructions on how to measure the gap in the combustion chamber in the factory workshop manual.

However there is really no need to check compression if the engine starts well and runs without excessive smoke.

 

[Old Bumbulum]

Another cause of low compression is a bent con rod. There are instructions on how to measure the gap in the combustion chamber in the factory workshop manual.

Indeed, and this is my fear. However in order to get the fuse wire inside the manual glibly tells you to remove the injector, then remove the pre- combustion chamber but gives no inkling as to how, nor is there a diagram of what the precombusion chamber consists of. Presumably it comes back up the injector aperture but how it is drawn out is beyond me. Flummoxed. The little watering-can like roses in the cylinder head of my previous 3GM did not seem to be removable even with the head off.

Robbieg - I thought that too but the Yanmar injector does not appear to have any threads inside to take an adaptor. In any case I don't have one.

I am coming round to the idea that if it runs OK it probably is OK but not very happy to have to guess rather than check!